A Future Without Feedback? Page 2

Engineering can explain much about the audio world. However, it is when it can't explain something that the real fun begins. Some aspects of perceived sound quality are not explained by established theory. There is a growing suspicion that some of these aspects---a loss in natural timbre; a duller, less expressive performance; increased aural fatigue; and missing life and energy in reproduced sound---may be consequences of the application of negative feedback.

It would be a mistake to demonize any particular philosophy. To do so forces people into entrenched positions and encourages the adoption of unhelpful defensive reactions, thus missing the opportunity for constructive dialog. Consider some of the contentious subjects that have been debated recently in these pages: analog vinyl and tape vs digital; one-box CD players vs separate transports and DACs; tubes vs solid-state; single-ended vs push-pull amplifiers; monoblock vs stereo amplifiers; class-A vs class-A/B or any other class of amplifier operation; pentodes vs triodes; and integrated amps vs pre- and power amps. Audiophiles have debated the merits or otherwise of cables ad nauseam. And in the high country of the tube purists, discussions rage about the virtues and vices of various types of triodes, even individual brands of triodes.

In my opinion, such debates have been valuable, though in some cases the importance of what differences do exist has been blown up out of all proportion. But the fact that a given difference or sonic error is detectable and audible doesn't mean that all is lost, that you can't adjust to and live with some of these transgressions.

Many of us working in the audio industry have long been aware that measurements do not fully describe sound quality. Moreover, it seems that measurements fail to describe some of the more important aspects of subjective perception. For example, we may guarantee that an amplifier will have a perfectly flat frequency response under normal conditions of use, yet we cannot explain why it may still sound duller or brighter than another comparably "flat" amplifier. We can measure crosstalk, channel separation, distortion, and noise to incredibly low levels, yet we cannot explain why some amplifiers have greater perceived stereo depth, resolution of detail, and low-level ambience than others. While we know that 0.3-0.5% of third-harmonic distortion is just audible in the midrange, how can the overall sound of a tube amplifier be judged "just fine" when we can measure 1.5% of second harmonic and 0.8% of third at a moderately high listening level?

Still more intriguing is the matter of dynamics. Some electronics sound flattened and dulled in terms of musical expression; others may be wonderfully revealing of this quality even at quiet sound levels. Or consider rhythm and timing: One power amplifier gets your foot tapping, another leaves you reading the sleevenotes. I can identify no measurement associated with rhythm or musical dynamics.

How can an amplifier produce superbly low measured treble distortion, yet give the aural impression that there's sand in the tweeter? How can an amplifier that in theory should have great low frequencies (for example, it has a DC-coupled topology, a big power supply, and a high damping factor) have soft, slow-sounding bass?

Some engineers have been developing an awareness of how we've gotten some of it wrong. Glimpses of audio heaven have been observed and reported with a number of exotic single-ended creations. More precisely, the SE units' sound over the broad midrange---in point of fact, over most of the significantly audible frequency range---reaches a level of purity and intrinsic musicality that inspires near-religious fervor. Such quality shows the rest of the industry what they're missing.

This isn't the place to expand upon the SE power-amplifier technology's strengths and, in some instances, audible weaknesses. Suffice it to say that the problems, often located at the extremes of the audio range, are in this context relatively harmless, and won't confound the following argument.

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